1. Field of the Invention
The present invention relates to a focus detecting device for detecting a focus condition of a photographic lens and an auxiliary illuminating device therefor, and more particularly to a TTL type focus detecting device which detects the focus condition of the photographic lens by receiving light passed through the photographic lens, and an auxiliary illuminating device suitable therefor.
2. Description of the Prior Art
In such TTL type focus detecting device, it becomes difficult or impossible to detect the focus condition of the photographic lens, if an object to be focused is insufficient dark or has an insufficient low contrast. U.S. Pat. No. 4,690,438 proposes an auxiliary illuminating device attachable to a camera for illuminating the object with projecting a projection pattern to assert the focus detection in such a case.
On the other hand, a multi-area focus detecting device is recently proposed in which the focus condition of the photographic lens is detected with respect to a plurality of focus detecting areas. The plurality of focus detecting areas includes at least one axial focus detecting area located on an optical axis of the photographic lens and also at least one off-axial focus detecting area located away from the optical axis of the photographic lens. In such multi-area focus detecting device, the distance between the off-axial focus detecting area and the optical axis of the photographic lens is changed in accordance with the change of the focal length of the photographic lens. In details, the distance is increased in accordance with the decrease of the focal length of the photographic lens, and is decreased in accordance with the increase of the focal length thereof. Therefore, the auxiliary illuminating device must illuminate a wide range extending from the axial position to the off-axial position with projecting the projection pattern, if it is used with the multi-area focus detecting device.
However, the conventional auxiliary illuminating device can not illuminate the off-axial position with sufficient intensity of light for the focus detection, even if it can illuminate the axial position with a sufficient intensity of light. Therefore, the sufficient function of the multi-area focus detecting device can not be performed, if the object is too dark or has an insufficient low contrast.
Additionally, in the conventional auxiliary illuminating device, no consideration has been given to a focus condition of a projection pattern image formed on the off-axial focus detecting area. Here, if a projection lens, provided for projecting the projection pattern onto the object, is constructed by one single lens element, it becomes difficult to well correct the spherical aberration of the projection lens and the astigmatism thereof with keeping the good balance, although it would be possible to control only the spherical aberration thereof within a proper range by means of introducing one or two aspheric surfaces to the projection lens. If the spherical aberration is not corrected well, the projection pattern image is blurred on the object. On the other hand, if the astigmatism is not corrected well, the focus position of the projection pattern image formed away from the optical axis is deviated from that formed on the optical axis. It causes that the range of distance within which the sufficient contrast for the focus detection can be projected differs depending on the projection pattern. Both of the spherical aberration and the astigmatism may be corrected well by means of constructing the projection lens by a plurality of lens elements. However, such construction results in a complex construction and an increase in both size and manufacturing cost.
Furthermore, the resolving power of the focus detecting device is improved in accordance with the increase of the focal length of the photographic lens, since the size of the focus detecting area is decreased in accordance therewith. Contrary, the resolving power of the focus detecting device is deteriorated in accordance with the decrease of the focal length of the photographic lens, since the size of the focus detecting area is increased in accordance therewith. Therefore, if the auxiliary illuminating device is used with the multi-area focus detecting device, it is necessary for designing the projection pattern so that the sufficient contrast for the focus detection should be provided with respect to any focal length of the photographic lens. In the above-explained auxiliary illuminating device, a plurality of thick stripes arranged at a relatively long pitch and a plurality of fine stripes arranged at a relatively short pitch are mixed with each other to form a projection pattern to be projected. However, the performance of the multi-area focus detecting device has not been fully exhibited by such projection pattern, since no consideration has been given to such multi-area focus detecting device in the auxiliary illuminating device.